Comparison of the electronic effect and the steric influence between the 1,1,2,2,2-pentamethyldisilanyl and the trimethylsilyl groups

A comparison of the electronic effects expressed by the Me3SiMe2Si-; and Me3Si- groups towards carbocationic intermediates was made by study of acid-catalyzed rearrangements of decalin epoxides 3 and 4. Reaction of the Me3SiMe2Si-epoxide 3 with BF3 · Et2O in CH2Cl2 at -50°C gave four products: B-ring contracted spirocyclic alcohol 9 (21%), methyl-migrated decanol 12 (23%), A-ring contracted aldehyde 15 (21%), and fluorohydrin 17 (14%). Under the same conditions, Me3Si-epoxide 4 afforded alcohol 9 (24%), decanol 12 (18%), aldehyde 16 (23%; cf. 15), and fluorohydrin 18 (13%; cf. 17). These results show that the Me3SiMe2Si- and Me3Si- groups exerted electronic influence to the same extents on carbocations. In HMPA and THF at -78°C, reagent Me3SiMe2SiLi reacted with cross-conjugated dienone 19 in a 1,4 fashion from the less hindered side to give enone 20 (63%) as the only adduct. In comparison with Me3SiLi, reagent Me3SiMe2SiLi possessed better stereoselectivity. In addition, the Me3SiMe2Si- moiety with α configuration in 24 was able to prevent m-chloroperbenzoic acid (m-CPBA) from attacking from the same face during epoxidation, such that β-epoxide 3 was obtained in 59% yield exclusively.